import pandas as pd
df = pd.DataFrame({'商品':['苹果','香蕉','梨'],'单价（元）':[5,5,4],'销量(kg)':[100,None,None],'库存(kg)':[50,None,70]})
print(df)
print(df.isnull())
print(df.notnull())
print(df.isnull().sum())